<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>优惠券</title>
    <style>
        .coupon {
            width: 200px;
            height: 100px;
            background-color: red;
            mask: radial-gradient(circle at left center, transparent 20px, red 20px);
            -webkit-mask: radial-gradient(circle at left center, transparent 20px, red 20px);
        }
    </style>
</head>
<div class="coupon">

</div>

<body>

</body>

</html>